Inorganic fibers are composed mainly of fibers comprising an inorganic compound. They are expected to be used as a constituting material for a heat insulating material, a refractory material, or the like. In particular, inorganic fibers having an average fiber diameter of 1 μm or less are expected to be used as a filter material or a sealing material.
As for inorganic fibers having a small fiber diameter, asbestos have conventionally be known. Since asbestos have a small fiber diameter and have a high chemical resistance to body fluids, it reaches the inside of the lung by breathing, and adversely affects the cells in the alveolus for a long period of time, thereby affecting the human body. Further, as for inorganic fibers other than asbestos, in the case of inorganic fibers composed mainly of SiO2, which have excellent heat resistance, it is known that SiO2 is crystallized at high temperatures to generate crystobalite, and the crystobalite reaches the inside of the lung, thereby adversely affecting the human body.
Under such circumstances, as inorganic fibers having low chemical resistance to body fluids, inorganic fibers comprising 65 to 99 wt % of Al2O3 and 1 to 35 wt % of SiO2 are proposed. These inorganic fibers are obtained by drawing a spinning solution formed of a solution comprising aluminum oxychloride, colloidal silica and a thickening agent from a plurality of spinning holes to obtain a fiber precursor, and the fiber precursor is then subjected to quick heating and quick quenching (see Patent Document 1 (U.S. Pat. No. 3,979,494)).